Lecture 4.10: Mars -- a habitable environment

Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.

教学方

Mike Brown

Professor

脚本

We're now in a position to go back and look at some of those results that John Grotzinger originally showed us back when we talked about Mars. And really look at the environment where the Mars Curiosity Rover has been, and examine it for habitability. This was the main mission, still is the main mission, of the Mars Curiosity Rover to find previously habitable environments on Mars. And early on in the mission they found one that by all measures, fits the bill. Previously habitable, let's understand now how that works. First I'm going to remind you where we are on Mars, this is my favorite Mars website once again. Let's go and zoom in on Gale Crater where Curiosity has landed. It's easy to find. Here's our favorite things, Tharsis Rise, volcanoes, Olympus Mons here, Valles Marineris. Gale Crater is along this dichotomy boundary and in the MOLA data it sticks out, it's that one right there. And it just sort of sticks out because of its color in this particular one but it's an interesting looking crater as we zoom in, zoom, zoom, zoom. Continue to zoom in. You got a nice flat bottom. The bottom is actually lower than most of the other areas around it, not a bad place to look if you're looking for something that once had water on it. Zoom in a little bit more. Zoom in a little bit more and as you remember there are river channels that go in on this side of it And there's one that comes in on this side. The landing was right around here. This is Mount Sharp where the rover is eventually headed. It's got nice layers that they're going to try to map out. But it landed over here in the flat region. Which is something that landers like to do. To be a lot safer. Okay, we zoom in more on the MOLA and you stop being able to see detail. But one of the really nice ways to look at it is with the THEMES Daytime Global Mosaic. And then you see this spectacular looking crater that we're zooming in on here. Let's zoom in some more and you'll start to see where Curiosity came in. See, this river going down in through here, or I say river channel going down in through here. But this channel going in through here Has this feature that's here, goes like this. This is clearly an alluvial fan that is from material that was brought down by this channel. Where did this channel come from? Well, you can see it coming in from up here and up in here. All of this stuff is down below the level up here. So water would have flown in like this, bringing materials with it. Where does it go? Well, this is what John Grotzinger was originally talking about when he said, walk downhill and follow the water. That's what Curiosity did. The channel comes in like this. The alluvial fan remains like this. And these are regions that are low and presumably would have pooled whatever water was there. Where did Curiosity land? Just about, as I look at this picture and I see this this little monster there's its eye, there's its mouth and Curiosity landed just on this side of the little monster. Here's now a geological map of where the landing area was and this comes from the, I think, just beautiful paper by John Grotzinger in Science in 2014, that describes the region where the rover went next. I do actually recommend reading this, the link to it's on our website, it's a free download from Caltech. It really, to my mind, counts as a pretty spectacular discovery in this history of looking for habitable environments throughout the universe. But first, let's see what Curiosity did. If you look really carefully you can see the monster again, you can see it by where they mapped out geologically BF, bedded fracture unit. Bedded fracture unit, we're going to talk a lot about that bedded fracture unit in a minute, but you can see this comes out like this. And like that, there's the mouth of the monster. And you can see where Curiosity landed right there, almost next to the bedded fracture unit. But instead inside this green stuff, hummocky plains. Down through here and also very close to this red stuff, cratered surface. Cratered surface you might guess is an old surface. Hummocky plains, you might guess, who knows what you might guess. Bedded fracture, the bedded part of the bedded fracture unit is the important part. Bedded means rock layers. Rock layers form in sediments, when you have sediments laying down on top of each other. Sediments form in aqueous environments. Let's look at that region in a little bit more detail. You saw an almost identical picture from, again from John Grotzinger when he was showing you where Curiosity landed. And Curiosity landed right here, it's called the Bradbury Landing Site. And you can really see where the rockets from the landing blasted away the dust. And this whole region has been scoured of all the dust around it. So it looks very different from the rest of the surface. And again John Grotzinger talked about this triple junction. So here you see we're in the hummocky plains. There is the bedded and fractured zone up through here and then the ancient cratered zone in through here. Ancient cratered zone, may be interesting maybe not, but bedded fracture: clearly interesting. You can see a little bit of the bedding already from here. You can see these lines like this and more lines in through here like this. You can sort of see them in through here like this. This is a picture from space, you're looking at those layers as they've been cut and carved away. Here you see a crater and you see dunes inside there. Don't get fooled into thinking those are bedded. But there's an important point, which is, from the spot where it was landed to this junction of these three different types of materials is downhill. Again, John Grotzinger said, go downhill and follow the water. Here's the elevation at the landing site -4500. The minus doesn't really mean anything it's just an arbitrary definition but it goes from -4500 down to -4420. Goes down 20 meters across this 500 meter distance from here to here. So, it's a nice little downhill slope and looking at this image, knowing what was in this direction of course they wanted to drive in this direction to go start looking at what was going on on Mount Sharp. But they realized that only 500 meters away was this bedded region that was at the base of an alluvial fan. It would be crazy not to go check it out, and so they did. You can see the driving, drive, drive, drive, drive, drive, each one of these little dots is a stop on a single day. And finally got to this little region down in through here, which you can now see better in here going down into this region here. Mostly going down into the bedded stuff, not really worrying too much about the ancient cratered surface, in fact never even going on the ancient cratered surface. And what did they see? Here's a spectacular view I think, from actually having driven down into that depression, where all the bedding is, and actually looking back out over it. So the landing site, I don't know, must have been somewhere way over here. The Mount Sharp to which they're eventually driving is up in here, the foothills of Mount Sharp and the cratered unit would be somewhere over in here. But here we are in the middle of the bedded formation and you can see it, you can see that these things, these bed plains, sometimes they stick up, sometimes they have collapsed down a little bit. But you can clearly see them. If you just were dropped in this location on the Earth and you started walking around, I think you would very quickly come to the same conclusion that the Curiosity team came to which is that they are looking at, a former lake. Now, they had a good reasons for even expecting it. It was at the bottom of this alluvial fan. As they were driving from the landing site to this location they found these conglomerates, this evidence of water flowing, of a river channel through there. And then they get to this location, there is this flat bottoms, multiple layers and they found, particularly in this region called a Sheepbed, this material that we are sitting on right now as we're taking this picture. This material is a mudstone. How do you get a mudstone? Well you can probably guess how you get a mudstone, first you make mud then you cover it up and it turns into stone. How do you make mud? Yeah, water, lakes. So we're sitting on a former lake, which is a pretty spectacular thing to be sitting on. That's great, but we were pretty sure that there were regions that were former lakes on Mars. In fact, we had looked at the Meridiani Planum from the previous rover and found clear evidence of interaction with liquid water, sometimes surface, sometimes ground water. But what's interesting here is the composition of these rocks. How do we know the compositions of these rocks? This is from the drilling. These are the drill sites. You can see what they named all these individual things. We're going to look at data from the John Klein drill site which is right there. Here are results; it's from the same issue of Science that all these other results on Yellowknife Bay, which is the name the team gave to this little area. And you saw these plots previously, again, when John Grotzinger was showing them. But I'm going to show you the actual data that it came from, the paper itself. And let's talk about it a little bit more. So what did they do? They took a sample, they drilled, they took that material, they put it into basically an oven and slowly heated that sample up. As that sample heats up, different gases come off as you start to break down whatever the minerals are inside the rock. What do you see a lot of? You see a lot of water. That's what this one is here, H2O. How do you know, I should have mentioned. How do you know what you're seeing? Because you take that gas that comes off and you stick it through a mass spectrometer. A mass spectrometer measures the molecular mass of the materials that you're doing. And, so, you can very clearly identify exactly what you have in there, with some issues that we won't go into. So, what do you see? You see mostly water. As you heat it up just a little bit the water comes off very quickly which tells you that it's not very heavily bonded into the structure of the mineral. And it continues to come off for the entire time. There's an extra little peak right here, which I find interesting. What else do you see? Little oxygen comes off, goes down, comes back up again. Those double peaks in something like oxygen tell you again that you're seeing two different places, two different ways that that oxygen is chemically mineralogically bound into whatever the minerals are that you're looking at. CO2, none of these things are particularly surprising that you see through here or, or worth jumping up and down about. If you want to jump up and down, here's why. This peak right here SO2, SO2 is a key one. Why is SO2 key? Well, we've seen that here on the Earth microbes can harness SO2 as their energy source. What's more is you continue to heat it up more, H2S. We definitely have talked about H2S, as an important source for certain microbes and a certain type of microbe would certainly make use to the H2S and the HSO2, that's here. There are other critical points that are discussed in this paper. One, is that they addition to measuring just the evolved gasses the overall chemistry of this region is not particularly acidic. If you remember, Meridiani Planum was acidic like that Rio Tinto in Spain, a big sulfuric acid environment. This is not, this is a very neutral pH. With abundant materials that can be harnessed by life. Clear evidence of both flowing water and a lake and some indication that this lake environment lasted for a long time. It's hard to know exactly how long in the paper. Grotzinger and his team tried to estimate it by saying, look, we don't really know how long the mudstones take to build up, the mud takes to build up. But if we make a gross estimate of how long that layer of mud took to form, you come up with may be a couple of thousand years. You could be wrong by factor of 10 or more in either direction but certainly 100 years, now 100 years is not a very long time. If you look from space, it looks like there's a lot more than just that tiny amount that they were looking at in that tiny section there. And they estimate that there could have been water flowing in these environments for a million years. Maybe not continuously, but at least sporadically. Interestingly, this is early Hesperian time period, this is not Noachian. We're used to the idea that there was water flowing in the Noachian. We are less used to the idea that in the Hesperian, which is when we think of as Mars drying out and forming these sulfuric acidic regions like Meridiani Planum. We're not used to there being long periods of waterflow like it looks like we had here. Is this an unusual environment? Perhaps, we certainly know that when we look globally that it is true that the Hesperian is a time of drying out of the planet. But in certain regions, it was clearly not happening very quickly. All these findings lead to the conclusion which is easily seen in the title of this paper. A Habitable Fluvio-Lacustrine, fluvio means river, lacustrine means lake. So it's the rivers going in a big lake region there. Environment at Yellowknife Bay, Gale Crater, Mars. Let's read the abstract of this and see if we can understand what all these things mean. Discovered fine-grained sedimentary rocks, this is the mudstone that's a very important component of knowing that there was a lake there. And it would have been suited to support a Martian biosphere founded on chemolithoautotrophy. Let's think of those words, chemo, chemical, that one we know. Litho, rock, auto, self, trophy, growth. Growth of microbes that are getting chemicals from rocks, which is enough for them to grow. Environment had a minimum duration of hundreds to tens of thousands of years, possible duration of million years and post-Noachian meaning during the Hesperian. What does this tell us? This tells us that Mars had a habitable environment in the past. Maybe not all of Mars. Maybe just small regions of Mars. But if it had a habitable environment in the past it begs the very, very obvious question, was it inhabited in the past? Interestingly, the Curiosity Rover was not designed to answer that question. It was very specifically only to answer the, was there a habitable environment? People were felt burned by the Viking results. When Viking results came back ambiguous or negative, nobody wanted to talk about they're looking for life on Mars anymore. But instead wanted to look for previously habitable environments. This is a previously habitable environment. And now, the conversation has changed. Theres the obvious question that now really should be asked in this previously habitable environment, was that environment inhabited? How would you find out? Well now, you would like to go back to this place or a place much like it and start to look not just at the habitat but look for evidence of organic molecules of the sorts that microbes build. None of the instruments onboard Curiosity could do that very well. And this is a big task of the next rover that's proposed to fly to Mars 2020. Rover Mars 2020 is a duplicate of Curiosity, at least the rover part of it. It's going to have brand new instruments on it though, that will presumably specifically look for evidence of past life, not just past habitability. It's a pretty exciting time in understanding habitability in the universe when we have really demonstrated beyond a doubt, I think it's fair to say that at least one other planet in our solar system was absolutely habitable at some point in the past. All these results that I was just telling you about came within the first year of the mission of Curiosity. You remember it landed here at this Bradbury landing site and all of that stuff that we are talking about came from Yellowknife Bay in that first year. Curiosity has now been on Mars for more than 1,600 Martian days. This is an update from early 2017, and in that time it has gone nearly nine miles. It went down to Yellowknife Bay and then went, traversed along here looking for a spot where it can get into Mount Sharp. Mount Sharp is off in this direction and after a long time of traversing this way it finally has shot straight in. Along the way there have been pretty exciting discoveries. One of the most interesting ones to me is that although the muds at Yellowknife Bay, and now that's been reproduced in many places in through here, certainly looked like there was a lake, it looked like it was a region that was habitable. The one thing that has not been found anywhere along through here has been evidence of carbonates, like for example calcium carbonates. Why do we care? Well, if this were a warm, habitable time in the history of Mars, one thing you would really expect is that the atmosphere had a lot of CO2. That's the way we think that Mars would have been able to sustain a greenhouse effect large enough to have had a warm environment where water could have been stable here on the surface in these areas down through here. Where did the CO2 go? Well we've talked about that but one of the places where CO2 might have gone hiding is into these calcium carbonates. And when we look at these clays and at these rocks in through here, we should see them. There aren't any signatures of global calcium carbonate deposits on Mars. There are a couple small ones, but really not enough to account for anything. So people have thought maybe it's because it's all embedded in all these rocks everywhere, and we'll just find it when we look in detail. The answer, no, no calcium carbonates. So to me this is actually one of the more interesting mysteries that Curiosity has comes across. I want to leave you with this beautiful 360 degree panorama that the Curiosity rover took as it was just beginning the ascent of Mount Sharp. And by now you probably recognize these sort of things. They look like those mud stones, they're flattened, they look like a place where probably there was a lake. There are these buttes that's sticking out here which means that these material on top is harder to erode. And so it's going to be a different, you can see them out through here, a different type of material. And Mount Sharp where Curiosity is heading, it's off here in the distance. Where Curiosity has been, because this is a 360 degree panorama, where Curiosity has been, it's been down here in the flat lands. The story of Curiosity so far has been all of these mudstones and this lake-like, habitable environment. As Curiosity goes up onto Mount Sharp, it's going to get more into those regions where the sulfates are more common. And it'll start to examine that transition from these wet conditions to these acidic dry conditions. And really teach us a lot more about the history of Mars and the history of habitability on that fascinating planet.